1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell, a cathode gas supply apparatus, an anode gas supply apparatus, an anode gas replacement apparatus, and a dilution apparatus. Further, the present invention relates to a method of starting operation of the fuel cell system.
2. Description of the Related Art
Fuel cells are systems for obtaining direct current electrical energy by electrochemical reactions of an anode gas (chiefly hydrogen-containing gas) supplied to an anode and a cathode gas (chiefly oxygen-containing gas) supplied to a cathode.
For example, a solid polymer electrolyte fuel cell includes a power generation cell formed by sandwiching a membrane electrode assembly between separators. The membrane electrode assembly includes the anode, the cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. In use of this type of power generation cell, generally, predetermined numbers of the membrane electrode assemblies and separators are stacked alternately to form a fuel cell stack, and the fuel cell stack is mounted in a vehicle such as an automobile.
In the fuel cell of this type, when power generation (operation) is stopped, supply of the anode gas and the cathode gas to the fuel cell is stopped. However, some anode gas remains at the anode, and some cathode gas remains at the cathode. Therefore, while operation of the fuel cell is stopped, the cathode gas (air) from the cathode moves through the electrolyte membrane to the anode, and thus, the cathode gas is present at both of the cathode and the anode.
In this regard, a method of starting operation of a fuel cell apparatus disclosed in Japanese Laid-Open Patent Publication No. 11-097047 is known. This conventional technique is intended to provide a method of starting operation of the fuel cell apparatus in which operation of the fuel cell apparatus is restarted rapidly even if power generation is stopped for long time. The technique relates to a method of starting operation of a sealed hydrogen type fuel cell apparatus including a fuel cell body, a hydrogen storage tank for storing (occluding) hydrogen required for the fuel cell body, a pressure control unit for controlling the pressure of the hydrogen supplied from the hydrogen storage tank to the fuel cell body, a hydrogen control valve for controlling the flow of the hydrogen, air supply means for supplying oxygen required for power generation of the fuel cell to the fuel cell body, a control unit for controlling the air supply means, a discharge valve provided on the anode side of the fuel cell, a switch for controlling an external output from the fuel cell body, and a control unit for monitoring the output voltage of the fuel cell and controlling the discharge valve and the switch.
At the time of starting operation of the fuel cell body, the hydrogen control valve for controlling the flow of the hydrogen from the hydrogen storage tank is opened. Then, in response to a signal from the control unit for controlling the air supply means, the air supply means supply the air to the fuel cell body. Then, the discharge valve is opened. After the output voltage of the fuel cell becomes a certain voltage or more, in response to a signal from the control unit for controlling the discharge valve, the discharge valve is closed to start the external output.
In the above conventional technique, after the pressure on the anode side is increased by opening the hydrogen control valve, the air is supplied to the fuel cell, and then, the discharged valve is opened to discharge the remaining gas which does not induce reaction. At this time, the hydrogen as the anode gas cannot be discharged directly, and needs to be diluted with the air as the cathode gas.
In this case, the amount of hydrogen discharged from the fuel cell through the discharge valve is subject to change depending on the change in a stoppage time (i.e., a period of time from stopping of operation of the fuel cell system to starting of operation of the fuel cell system). It is because, the anode gas is consumed by chemical reaction when operation is stopped, and the amount of the remaining hydrogen is reduced due to out-leakage or cross-leakage.
However, in the above conventional technique, regardless of change in the amount of the remaining hydrogen, the amount of the air for diluting the hydrogen discharged from the fuel cell is regulated to the maximum amount which is required for dilution. Therefore, at the time of diluting the hydrogen, the air is supplied excessively, and consumption of energy needed for operating the compressor (pump) is increased uneconomically.